1. Field of the Invention
The present invention relates to a luminous device using a luminous element which has a film containing an organic compound (hereinafter referred to as an “organic compound layer”) between a pair of electrodes and which can give fluorescence or luminescence by receiving an electric field. The luminous device referred to in the present specification is an image display device, a luminescent device or a light source. Additionally, the following are included in examples of the luminous device: a module wherein a connector, for example, a flexible printed circuit (FPC) or a tape automated bonding (TAB) tape, or a tape carrier package (TCP)) is set up onto a luminous element; a module wherein a printed wiring board is set to the tip of a TAB tape or a TCP; and a module wherein integrated circuits (IC) are directly mounted on a luminous element in a chip on glass (COG) manner.
2. Related Art
A luminous element is an element which emits light by receiving an electric field. It is said that the luminescence mechanism thereof is based on the following: by applying a voltage to an organic compound layer sandwiched between electrodes, electrons injected from the cathode and holes injected from the anode are recombined in the organic compound layer to form molecules in an exciting state (hereinafter referred to as “molecular excimers”); and energy is radiated when the molecular excimers return to the ground state thereof.
The kind of the molecular excimers which are made from the organic compound may be a singlet exciting state excimer or a triplet exciting state excimer. In the present specification, luminescence (that is, light emission) may be based on the contribution of any one of the two.
In such a luminous element, its organic compound layer is usually made of a thin film having a thickness below 1 μm. The luminous element is a spontaneous light type element, wherein the organic compound layer itself emits light. Therefore, backlight, which is used in conventional liquid crystal displays, is unnecessary. As a result, the luminous element has a great advantage that it can be produced into a thin and light form.
The time from the injection of carriers to the recombination thereof in the organic compound layer having a thickness of about 100 to 200 nm is about several tens nanoseconds in light of carrier mobility in the organic compound layer. A time up to luminescence, which includes the step from the recombination of the carrier to luminescence, is a time in order of microseconds or less. Therefore, the luminous element also has an advantage that the response thereof is very rapid.
Since a luminous element is of a carrier injection type, the luminous element can be driven by DC voltage and noises are not easily generated. About driving voltage, a sufficient brightness of 100 cd/m2 is attained at 5.5 V by making an organic compound layer to a super-thin film having a uniform thickness of about 100 nm, selecting an electrode material so as to make a carrier injection barrier against the organic compound layer small, and further introducing a hetero-structure (bilayer structure) (document 1: C. W. Tang and S. A. VanSlyke, Organic electroluminescent diodes, Applied Physics letters, vol. 51, No. 12, 913–915 (1987)).
In light of such properties such as thinness and lightness, high-speed response, and DC low voltage driving ability, attention is paid to a luminous element as a flat panel display element in the next generation. Since the luminous element is of a spontaneous light type and has a wide field angle, the luminous element is relatively easy to watch. Thus, it can be considered that the luminous element is effective as an element used in a display screen in portable devices.
In luminous devices formed by arranging such luminous elements in a matrix form, driving methods called passive matrix driving (simple matrix type) and active matrix driving (active matrix type) can be used. However, in the case in which the density of pixels increases, it is considered that the active matrix type wherein a switch is fitted to each pixel (or each dot) is more profitable since lower voltage driving can be attained.
Incidentally, in such a luminous element, a metal material having a low work function is used as a cathode since electron injection is facilitated. Hitherto, the following have been investigated as materials satisfying practical properties: magnesium alloy such as alloy of Mg and Ag, and aluminum alloy such as alloy of Al and Li. All of the material systems are easily oxidized by water content in the atmosphere, so that a dark spot, which is a luminescence defect of the element, is generated or a rise in voltage occurs. Therefore, a form using some protective film or some sealing structure is necessary as a final form of the element.
In light of the background art of the above-mentioned alloy electrodes, it has been desired to develop more stable cathodes. In recent years, it has been reported that by interposing a cathode buffer layer made of lithium fluoride (LiF) or the like as a super-thin insulating layer (0.5 nm), even an aluminum cathode can give luminescence property equivalent to or more than that of alloy of Mg and Ag, or the like alloy (document 2: L. S. Hung, C. W. Tang and M. G. Mason: Appl. Phys. Lett,. 70(2), 13 Jan. (1997).
The mechanism of the property improvement by disposing this cathode buffer layer would be as follows: when LiF constituting the cathode buffer layer is formed to contact Alq3 constituting an electron transport layer of an organic compound layer, the energy band of Alq3 is bent to lower an electron injection barrier.
As described above, in a luminous element composed of an anode, a cathode and an organic compound layer, an invention is made for improving the capability of injecting carriers from the electrode, resulting from an element characteristic of the luminous element.
Hitherto, a simple substance selected from elements belonging to group I or group II in the periodic table or a compound containing this substance has been used as a material having a small work function to form a cathode buffer layer between the cathode and the organic compound layer.
However, in the case in which any metal selected from alkali metals and alkali earth metals belonging to group I or group II in the periodic table is used alone for the cathode buffer layer, there arises a problem that the metal diffuses to have a bad effect on properties of a TFT connected to the luminous element.
On the other hand, in the case in which the compound containing any element selected from elements belonging to group I or group II in the periodic table is used for the cathode buffer layer, a compound of the element and oxygen, fluorine or the like belonging to group XVI or XVII in the periodic table, which has a large electronegativity, is generally used in order to make work function smaller. However, such a compound is non-conductive and thus electron-injection capability is improved. However, in order that the element characteristic is not deteriorated, it is necessary to make the film thickness of the cathode buffer layer as highly thin as 1 nm or less. Therefore, a scattering in the film thickness is easily generated in respective pixels, and it is difficult to control the film thickness.